Lubricant composition

ABSTRACT

A lubricant composition which, in a sliding member, is interposed between a DLC-coated surface and another metal member, in particular, a steel material, the lubricant composition having good wear resistance as well as having a better friction reducing effect than conventional molybdenum friction modifier-containing lubricant compositions. The lubricant composition is characterized by containing a lubricating base oil, (A) molybdenum dialkyl dithiophosphate, and (B) zinc dialkyl dithiophosphate, wherein the amount of phosphorus with respect to the total mass of the lubricant composition is 300-1500 mass ppm.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is the National Phase entry of InternationalPatent Application No. PCT/JP2018/003468 filed on Feb. 1, 2018, whichclaims priority to Japanese Patent Application No. 2017-016661 filed onFeb. 1, 2017, the entire contents of which are hereby incorporated byreference into this application.

FIELD

The present disclosure relates to a lubricant composition. Specifically,the present disclosure provides a lubricant composition for slidingmembers, more specifically a lubricant composition for sliding membersin internal combustion engines.

BACKGROUND

Lubricant compositions are now widely used in automobile fields, such asfor internal combustion engines, automatic transmissions, and gear oils.In recent years, lubricant compositions have been required to have lowerviscosities in order to improve fuel consumption. However, loweringviscosity may result in thinned oil films and thus increased boundaryfriction, leading to inability to sufficiently reduce friction.Reduction of friction is important for improvement of fuel consumption,and thus surface modification technologies for sliding members are nowreceiving attention. For example, various hard films are investigated asmeasures for reduction of friction and wear of sliding parts. Inparticular, various attempts are being made to develop those usingdiamond-like carbon (hereinafter abbreviated as “DLC”) films. Forexample, investigations are underway to obtain higher effect of reducingfriction by optimizing the combination of DLC film and lubricantcomposition disposed between sliding members.

Published Japanese Translation of PCT International Publication forPatent Application (Kohyo) No. 2014-513173 (Patent Literature 1)discloses a lubricant composition containing an oil soluble organicmolybdenum friction modifier and active sulfur of a surface activesulfur donor component, for use in reduction of friction and improvementof wear properties of DLC coating. As the oil soluble organic molybdenumfriction modifier, molybdenum dithiocarbamates are used. JapaneseUnexamined Patent Publication (Kokai) No. 2014-224239 (Patent Literature2) discloses use of DLC-coated surface doped with specific elements andan oil soluble molybdenum compound having a chemical structure oftrinuclear Mo for reduction of friction. Furthermore, JapaneseUnexamined Patent Publication (Kokai) No. 2016-216653 (Patent Literature3) discloses a lubricant composition containing a molybdenumdithiophosphate and an amine or amide friction modifier as essentialcomponents, which lubricant composition has low friction and wearresistance to DLC films, particularly DLC films comprising ahydrogenated amorphous carbon.

CITATION LIST Patent Literature

[Patent Literature 1] Published Japanese Translation of PCTInternational Publication for Patent Application (Kohyo) No. 2014-513173

[Patent Literature 2] Japanese Unexamined Patent Publication (Kokai) No.2014-224239

[Patent Literature 3] Japanese Unexamined Patent Publication (Kokai) No.2016-216653

SUMMARY Technical Problem

However, lubricant compositions containing a molybdenum dithiocarbamateor a trinuclear Mo compound as a friction modifier still does not havesufficient friction reducing effects. In addition, lubricantcompositions containing a molybdenum dithiocarbamate are effective forsliding contact between steel materials, but may increase the wearamount by sliding contact between a DLC film and a steel material.Further, use of a lubricant composition containing an amine or amidefriction modifier at a content described in Examples in JapaneseUnexamined Patent Publication (Kokai) No. 2016-216653 (Patent Literature3) onto a DLC film surface containing boron may lead to highercoefficient of friction and increased wear amount.

In view of the above, an object of the present disclosure is to providea lubricant composition which is provided between a DLC-coated surfaceand another metal member (in particular a steel material) of slidingmembers, and which lubricant composition has both a better frictionreducing effect than that of conventional lubricant compositionscontaining molybdenum friction modifiers and a good wear resistance.

Solution to Problem

In order to solve the above problems, the present inventors haveintensively studied to find that a lubricant composition containingmolybdenum dialkyldithiophosphate and zinc dialkyldithiophosphate cansolve the above problems, thereby completing the present disclosure.

The present disclosure provides a lubricant composition comprising alubricant base oil, (A) molybdenum dialkyldithiophosphate, and (B) zincdialkyldithiophosphate, and having a phosphorus content of 300 to 1500ppm by weight based on the total weight of the lubricant composition.

Further, the present disclosure provides lubricant compositions furtherhaving at least one characteristics of the following (a) to (h):

(a) the lubricant composition, which further comprises, or does notcomprise at all, (A′) at least one selected from the group consisting ofan amine friction modifier and an amide friction modifier at a contentof less than 0.1% by weight based on the total weight of the lubricantcomposition;

(b) the lubricant composition, comprising component (A) at a content of400 to 1300 ppm by weight in terms of ppm by weight of molybdenum basedon the total weight of the lubricant composition, and component (B) at acontent of 200 to 1000 ppm by weight in terms of ppm by weight ofphosphorus based on the total weight of the lubricant composition;

(c) the lubricant composition, for use in lubrication of diamond-likecarbon (DLC) films;

(d) the lubricant composition, for use in lubrication of opposed slidingsurfaces of sliding members, wherein the sliding members are a pair ofsliding members having opposed sliding surfaces which can move relativeto each other, and wherein at least one of the sliding surfacescomprises a surface coated with a diamond-like carbon (DLC) film;

(e) the lubricant composition of (c) or (d), wherein the DLC comprisesboron;

(f) the lubricant composition, which has a high temperature high shearviscosity (HTHS viscosity) at 150° C. of 1.4 to 2.9 mPa·s;

(g) the lubricant composition, which has a kinematic viscosity at 100°C. of 9.3 mm²/s or less;

(h) the lubricant composition, for use in internal combustion engines.

In the present disclosure, the sliding member means a pair of membershaving opposed sliding surfaces which can move relative to each otherwhile being in sliding contact with each other, such as a shaft and abearing, and a piston and a liner. The sliding members wherein at leastone of the sliding surfaces comprises a surface coated with adiamond-like carbon (DLC) film in (d) above means that the surface ofone of a pair of opposed members, in sliding contact with the othermember is coated with a DLC film. The lubricant composition for use inlubrication of opposed sliding surfaces means a lubricant compositionfor use in lubrication between opposed sliding surfaces of a pair ofmembers. The lubricant composition may be disposed between the opposedsliding surfaces.

Technical Effects

The lubricant composition of the present disclosure can reduce acoefficient of friction between sliding surfaces. In particular, thelubricant composition of the present disclosure can provide a lowercoefficient of friction in sliding surfaces between a DLC-coated surfaceand another metal member (in particular, a steel material) than alubricant composition only containing a molybdenum dithiocarbamate or atrinuclear Mo compound as a friction modifier, and also is excellent inwear resistance. The lubricant composition of the present disclosure canbe suitably used as a lubricant composition particularly for use ininternal combustion engines.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic view of a Block-on-Ring friction test.

DESCRIPTION OF EMBODIMENTS

The lubricant composition of the present disclosure will be described indetail.

Lubricant Base Oil

Any lubricant base oils may be used, including mineral oils, syntheticoils, and mixed oils thereof. Examples of the mineral base oils includelubricant base oils such as paraffin oils and naphthene oils, obtainedby subjecting crude oil to atmospheric distillation and vacuumdistillation and purifying the obtained lubricant oil fractions inappropriate combination of purification processes such as solventdeasphalting, solvent extraction, hydrogenolysis, solvent dewaxing,catalytic dewaxing, hydrogenation refining, sulfuric acid cleaning, claytreatment; and lubricant base oils obtained by isomerization anddewaxing of waxes obtained by solvent dewaxing. The kinematicviscosities of the mineral base oils at 100° C. are, but not limited to,1 to 6 mm²/s, or from 2 to 6 mm²/s, in order to obtain a lubricantcomposition having a low viscosity.

Examples of the synthetic base oils which can be used includeisoparaffins, alkylbenzenes, alkylnaphthalenes, monoesters, diesters,polyol esters, polyoxyalkylene glycols, dialkyl diphenyl ethers, andpolyphenyl ethers. GTL (Gas to Liquid) base oils, ATL (Asphalt toLiquid) base oils, BTL (Biomass to Liquid) base oils and CTL (Coal toLiquid) base oils can also be used, and the processes for using them asraw materials are described in U.S. Pat. Nos. 4,594,172 and 4,943,672.The kinematic viscosities of the synthetic base oils are notparticularly limited. Poly-α-olefins or α-olefin copolymers having akinematic viscosity of less than 6 mm²/s or more than 80 mm²/s at 100°C. can also be used. The kinematic viscosities of the synthetic baseoils are 1 to 6 mm²/s, or from 2 to 6 mm²/s, in order to obtain alubricant composition having a low viscosity.

The above-described base oils which can be used in combination may beused alone or in combination of two or more. When two or more are used,two or more mineral base oils; two or more synthetic base oils; and oneor more mineral base oils and one or more synthetic base oils can beused.

In order to obtain a lubricant composition having a low viscosity, thelubricant base oils as a whole have a kinematic viscosity at 100° C.from 1 to 6 mm²/s, from 2 to 6 mm²/s, or from 2.5 to 6 mm²/s.

(A) Molybdenum Dialkyldithiophosphate

The present disclosure is characterized in that the lubricantcomposition comprises molybdenum dialkyldithiophosphate (MoDTP) as acomponent. For example, MoDTP is a compound represented by followingformula (1).

Friction modifiers include organic molybdenum compounds, complexes of amolybdenum compound and a sulfur-containing organic compound or otherorganic compound, and complexes of a sulfur-containing molybdenumcompound such as sulfurized molybdate and an alkenylsuccinimide.Examples of the organic molybdenum compounds include a molybdenumdialkyldithiophosphate (MoDTP) and a molybdenum dithiocarbamate (MoDTC).Among them, a molybdenum dithiocarbamate (MoDTC) has been used. This isbecause MoDTP contains phosphorus as shown in formula (1). Becausephosphorus poisons three-way catalysts for exhaust gas purification, thecontent of phosphorus in a lubricant composition is regulated. Inaddition, since conventional lubricant compositions have often beenblended with an organophosphorus compound as anti-wear agents, amolybdenum dithiophosphate has been avoided from aggressive use in orderto reduce the content of phosphorus contained in a lubricantcomposition. From these reasons, a molybdenum dithiocarbamate (MoDTC)has been used as friction modifiers in conventional lubricantcompositions. However, as described above, a lubricant compositioncontaining a molybdenum dithiocarbamate (MoDTC) may cause significantwear between sliding surfaces of a DLC-coated surface and a steelmaterial. In addition, the friction reducing effect of the lubricantcompositions is insufficient. On the other hand, a lubricant compositioncontaining a molybdenum dithiophosphate, as compared with a lubricantcomposition containing a molybdenum dithiocarbamate, can give anexcellent friction reducing effect to a sliding surface having a DLCfilm and improve the wear resistance.

As described above, the lubricant composition of the present disclosurecomprises a molybdenum dialkyldithiophosphate and a zincdialkyldithiophosphate. The total content of phosphorus contained in thelubricant composition is from 300 to 1500 ppm by weight, from 400 to1400 ppm by weight, from 500 to 1300 ppm by weight, from 600 to 1200 ppmby weight, or from 600 to 1000 ppm by weight based on the total weightof the lubricant composition. When a molybdenum dialkyldithiophosphateand a zinc dialkyldithiophosphate are combined at such contents that thetotal content of phosphorus is within the above ranges, both an improvedeffect of reducing the friction between sliding surfaces and anexcellent wear resistance can be obtained without catalyst poisoning.

In formula (1) above, Rs are each independently a monovalent C₁₋₃₀hydrocarbon group. The hydrocarbon group may be linear or branched.Examples of the monovalent hydrocarbon group include linear or branchedC₁₋₃₀ alkyl groups; C₂₋₃₀ alkenyl groups; C₄₋₃₀ cycloalkyl groups; C₆₋₃₀aryl groups, alkylaryl groups and arylalkyl groups. When the monovalenthydrocarbon group is an arylalkyl group, the alkyl group is bound to anyposition. More specifically, examples of the alkyl group include methyl,ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl,undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl,heptadecyl and octadecyl, and branched alkyl thereof. In particular, insome embodiments, Rs are each independently C₃₋₈ alkyl groups. X₁ and X₂are oxygen atoms or sulfur atoms. In some embodiments, X₁ and X₂ areoxygen atoms. Y₁ and Y₂ are oxygen atoms or sulfur atoms. In someembodiments, Y₁ and Y₂ are sulfur atoms.

The content of the above (A) MoDTP in the lubricant composition of thepresent disclosure, in terms of ppm by weight of molybdenum based on thetotal weight of the lubricant composition, is from 400 to 1300 ppm byweight, from 500 to 1200 ppm by weight, from 600 to 1100 ppm by weight,or from 650 to 1050 ppm by weight. When the content of MoDTP is belowthe lower limit, insufficient effect of reducing the friction betweensliding surfaces may be obtained. When the content of MoDTP is above theupper limit, wearing between sliding surfaces is undesirably increased.

The lubricant composition of the present disclosure can comprise amolybdenum friction modifier other than (A), such as a molybdenumdithiocarbamate (MoDTC), and a trinuclear molybdenum compound asoptional components in combination with the molybdenumdialkyldithiophosphate (MoDTP). The content of the molybdenum frictionmodifier other than (A) is within the range not impairing the frictionreducing effect obtained by the present disclosure and the wearresistance, and may be adjusted as appropriate without exceeding thecontent of MoDTP.

As described above, conventional lubricant compositions containing amolybdenum dithiocarbamate (MoDTC) alone may provide an insufficienteffect of reducing the friction and cause a significant wearing betweensliding surfaces of a DLC-coated surface and a steel material. WhenMoDTC is used in combination with MoDTP, the friction between slidingsurfaces of a DLC-coated surface and a steel material can be furtherreduced, and the wearing therebetween can be reduced. The content ofMoDTC, in terms of ppm by weight of molybdenum derived from MoDTC basedon the total weight of the lubricant composition, is 100 ppm by weightor less, 70 ppm by weight or less, or 50 ppm by weight or less. Thelower limit of the content is, but not limited to, 1 ppm by weight ormore, 5 ppm by weight or more, or 10 ppm by weight or more. When thecontent of MoDTC is above the upper limit, wearing between slidingsurfaces may be undesirably increased.

The lubricant composition can comprise a trinuclear molybdenum compoundin combination with MoDTP, and thereby can further reduce the frictionbetween sliding surfaces as compared with a lubricant compositioncomprising a trinuclear molybdenum compound alone. However, a lubricantcomposition comprising a trinuclear molybdenum compound may result inlarger wearing even in combination with MoDTP. Thus, when the lubricantcomposition comprises a trinuclear molybdenum compound, the content ofthe compound is very small without impairing the effects of the presentdisclosure. More specifically, in terms of ppm by weight of molybdenumderived from the molybdenum dithiocarbamate based on the total weight ofthe lubricant composition, the content is less than 50 ppm by weight, 40ppm by weight or less, or 20 ppm by weight or less. The lower limit isnot particularly limited. For example, the content is 1 ppm by weight ormore, 5 ppm by weight or more, or 10 ppm by weight or more.

(A′) Amide Friction Modifier and Amine Friction Modifier

The lubricant composition of the present disclosure may furthercomprise, in addition to the above, at least one selected from the groupconsisting of an amide friction modifier and an amine friction modifierin less than a specific content, or may not comprise any of them at all.When the lubricant composition comprises an amide friction modifier oran amine friction modifier, the total content of them is 0.1% by weightor less, 0.05% by weight or less, or 0.01% by weight or less based onthe total amount of the lubricant composition. Use of a lubricantcompositions comprising an amide friction modifier or an amine frictionmodifier at a content not less than the above content, especially to DLCfilms doped with boron, may cause the coefficient of friction to becomehigher and the wearing to become larger. The contents of the amidefriction modifier and the amine friction modifier is as small aspossible, and some embodiments may not comprise any of them at all.

Any amine friction modifiers and amide friction modifiers may be used.Examples include alkylamines having a linear or branched alkyl groupwith a carbon number from 1 to 30, from 4 to 28, or from 6 to 25, suchas methylamine, ethylamine, and propylamine; alkenyl amines having analkenyl group with a carbon number from 2 to 30, from 4 to 28, or from 6to 25, which may be branched, such as ethenylamine, propenylamine, andoleylamine; alicyclic amines such as cyclohexylamine; alkylenediamineshaving an alkylene group with a carbon number from 1 to 30, such asbutylenediamine; polyamines such as pentaethylenehexamine; and mixturesthereof. Examples of the amide friction modifiers include saturatedfatty acid amides having a linear or branched alkyl group with a carbonnumber from 1 to 30, from 4 to 28, or from 6 to 25, such as ethanamideand propanamide; unsaturated fatty acid amides having an alkenyl groupwith a carbon number from 2 to 30, from 4 to 28, or from 6 to 25, whichmay be branched, such as oleamide and erucamide; and mixtures thereof.

(B) Zinc Dialkyldithiophosphate

The lubricant composition of the present disclosure comprises a zincdialkyldithiophosphate (ZnDTP (also referred to as ZDDP)). This compoundis known as an anti-wear agent for lubricant compositions and isrepresented by following formula (2).

In formula (2), R¹ and R², which may be the same or different, are eacha hydrogen atom or a monovalent C₁₋₂₆ hydrocarbon group. For example,the monovalent hydrocarbon group is a hydrocarbon group containing aprimary or secondary C₁₋₂₆ alkyl group; a C₂₋₂₆ alkenyl group; a C₆₋₂₆cycloalkyl group; a C₆₋₂₆ aryl, alkylaryl or arylalkyl group; and anester linkage, an ether linkage, an alcohol group or a carboxyl group.R¹ and R² each is a primary or secondary alkyl group with a carbonnumber from 2 to 12, a C₈₋₁₈ cycloalkyl group, or a C₈₋₁₈ alkylarylgroup, and may be the same or different. In some embodiments, R¹ and R²may be zinc dialkyldithiophosphates, and the primary alkyl group has acarbon number from 3 to 12, or from 4 to 10. The secondary alkyl grouphas a carbon number from 3 to 12, or from 3 to 10. A zincdithiocarbamate (ZnDTC) may also be used in combination.

The content of the zinc dialkyldithiophosphate in the present lubricantcomposition is such a content that the total content of phosphorussatisfies the above-described ranges based on the total weight of thelubricant composition. In some embodiments, the content is such that thecontent of phosphorus derived from the ZnDTP is from 200 to 1000 ppm byweight, from 300 to 900 ppm by weight, from 350 to 850 ppm by weight, orfrom 400 to 800 ppm by weight, based on the total weight of thelubricant composition. When the lubricant composition comprises the zincdialkyldithiophosphate at a content satisfying the above ranges, both animproved effect of reducing the friction between sliding surfaces and anexcellent wear resistance can be obtained without catalyst poisoning. Itis noted that the lubricant composition of the present disclosure maycomprise one of zinc dialkyldithiophosphates having a primary alkylgroup (Pri-ZnDTPs) and zinc dialkyldithiophosphates having a secondaryalkyl group (Sec-ZnDTPs) alone, or two or more of them in combination.When the lubricant composition comprises them in combination, thecombination ratio is not particularly limited. When combined with amolybdenum dialkyldithiophosphate, use of any of Pri-ZnDTP and Sec-ZnDTPcan equally result in both an excellent friction reducing effect andwear resistance. In some embodiments, the lubricant compositions mayinclude a Sec-ZnDTP, in which the content of phosphorus derived from theSec-ZnDTP is, but not limited to, from 200 to 1000 ppm by weight, from250 to 900 ppm by weight, or from 300 to 800 ppm by weight based on thetotal weight of the lubricant composition.

In combination with the zinc dialkyldithiophosphate, at least onecompounds selected from phosphate- and phosphite-phosphorus compoundsrepresented by following formulae (3), (4) and (5), and metal salts andamine salts thereof can also be used. However, the contents of thesecompounds are limited in such amounts that the total weight ofphosphorus in the whole lubricant composition satisfies the rangesdescribed above. For example, the total content is less than 0.1% byweight, less than 0.05% by weight, or less than 0.01% by weight, basedon the total amount of the lubricant composition. In some embodiments,these compounds are not contained at all.

In formula (3) above, R³ is a monovalent C₁₋₃₀ hydrocarbon group; R⁴ andR⁵ are each independently a hydrogen atom or a monovalent C₁₋₃₀hydrocarbon group; and m is 0 or 1.

In formula (4) above, R⁶ is a monovalent C₁₋₃₀ hydrocarbon group; R⁷ andR⁸ are each independently a hydrogen atom or a monovalent C₁₋₃₀hydrocarbon group; and n is 0 or 1.

In formula (5) above, R⁶ is as described above.

Examples of the monovalent C₁₋₃₀ hydrocarbon group represented as R³ toR⁸ in formulae (3), (4) and (5) include alkyl, cycloalkyl, alkenyl,alkyl-substituted cycloalkyl, aryl, alkyl-substituted aryl, andarylalkyl groups. In particular, the monovalent C₁₋₃₀ hydrocarbon groupmay be a C₁₋₃₀ alkyl group or a C₆₋₂₄ aryl group, a C₃₋₁₈ alkyl group,or a C₄₋₁₅ alkyl group.

Examples of the phosphorus compound represented by formula (3) aboveinclude phosphorous acid monoesters and (hydrocarbyl)phosphonous acidshaving one C₁₋₃₀ hydrocarbon group described above; phosphorous aciddiesters, monothiophosphorous acid diesters, and(hydrocarbyl)phosphonous acid monoesters having two C₁₋₃₀ hydrocarbongroup described above; phosphorous acid triesters and(hydrocarbyl)phosphonous acid diesters having three C₁₋₃₀ hydrocarbongroup described above; and combinations thereof.

As described above, the lubricant composition of the present disclosurecomprises a lubricant base oil, (A) a molybdenum dialkyldithiophosphate,and (B) a zinc dialkyldithiophosphate. The lubricant composition canfurther comprise an amide friction modifier and an amine frictionmodifier in less than the specific content described above, but, in someembodiments, the lubricant composition does not comprise them at all. Inaddition, the lubricant composition may comprise one or more selectedfrom (C) a viscosity index improver, (D) an ashless dispersant, and (E)a metal detergent, as optional components.

(C) Viscosity Index Improver

Examples of the viscosity index improver include so-callednon-dispersion viscosity index improvers such as polymers or copolymersof one or more monomers selected from various methacrylic acid esters,and hydrogenated products thereof; and so-called dispersion viscosityindex improvers obtained by copolymerizing various methacrylic acidesters including nitrogen compounds; non-dispersion or dispersionethylene-α-olefin copolymers (the α-olefin includes propylene, 1-butene,and 1-pentene), and hydrogenated products thereof; polyisobutenes andhydrogenated products thereof; hydrogenated products of styrene-dienecopolymers; styrene-maleic anhydride ester copolymers; star-shapedisoprenes; and polyalkylstyrenes. Furthermore, comb polymers comprisingat least a polyolefin macromer-based repeating unit and a repeating unitbased on alkyl (meth)acrylate having a C₁₋₃₀ alkyl group in the mainchain can be used.

The molecular weight of the viscosity index improver is selected inconsideration of the shear stability of the lubricant composition. Forexample, viscosity index improvers are used, which have a weight averagemolecular weight of usually from 5,000 to 1,000,000, or from 100,000 to900,000 for dispersion and non-dispersion polymethacrylates; usuallyfrom 800 to 5,000, or from 1,000 to 4,000 for polyisobutenes orhydrogenated products thereof; and usually from 800 to 500,000, or from3,000 to 200,000 for ethylene-α-olefin copolymers or hydrogenatedproducts thereof.

Among the viscosity index improvers, when an ethylene-α-olefin copolymeror a hydrogenated product thereof is used, a lubricant compositionhaving particularly excellent shear stability can be obtained. Any oneor more compound(s) selected from the viscosity index improversdescribed above can be contained in any content(s).

The content of the viscosity index improver in the lubricant compositionis from 0.01 to 20% by weight, from 0.02 to 10% by weight, or from 0.05to 5% by weight, based on the total weight of the composition.

(D) Ashless Dispersant

The lubricant composition of the present disclosure can further comprisean ashless dispersant. Any ashless dispersants may be used withoutlimitation. Examples of the ashless dispersant includenitrogen-containing compounds having at least one linear or branchedC₄₀₋₄₀₀ alkyl or alkenyl group in the molecules, and derivativesthereof, and succinimides and modified products thereof. The ashlessdispersants may be used alone or in combination of two or more.Boronated ashless dispersants can also be used. The boronated ashlessdispersants are obtained by boronation of any ashless dispersants usedin lubricants. Boronation is generally carried out by reacting an imidecompound with boric acid to neutralize a part or all of the remainingamino groups and/or imino groups.

The carbon number of the alkyl or alkenyl group described above is from40 to 400, or from 60 to 350. When the carbon number of the alkyl oralkenyl group is below the lower limit, the solubility of the compoundin a lubricant base oil tends to decrease. On the other hand, when thecarbon number of the alkyl or alkenyl group is above the upper limit,the low-temperature fluidity of the lubricant composition tends todeteriorate. The alkyl and alkenyl groups may have linear or branchedstructures. In some embodiments, a branched alkyl or alkenyl groupderived from an oligomer of an olefin such as propylene, 1-butene, orisobutene, or a co-oligomer of ethylene and propylene is used.

The succinimides include so-called mono-succinimides which are reactionproducts between one end of polyamines and succinic anhydride, andso-called bis-succinimides which are reaction products between both endsof polyamines and succinic anhydride. The lubricant composition of thepresent disclosure may comprise either one or both of the mono- andbis-succinimides.

The modified products of succinimides are, for example, those obtainedby modifying a succinimide with a boron compound (hereinafter alsoreferred to as boronated succinimide). Modification with a boroncompound refers to boronation. Boronated succinimides may be used aloneor in combination of two or more. When used in combination, two or moreboronated succinimides may be combined. Both a boronatedmono-succinimide and a bis-succinimide may be contained. Alternatively,boronated mono-succinimides may be combined, or boronatedbis-succinimides may be combined. A boronated succinimide and anon-boronated succinimide may be combined.

For example, methods for producing boronated succinimides include thosedisclosed in, for example, Japanese Examined Patent Publication (Kokoku)Nos. S42-8013 and S42-8014, and Japanese Unexamined Patent Publication(Kokai) Nos. S51-52381 and S51-130408. Specifically, for example, aboronated succinimide can be obtained by mixing an organic solvent suchas alcohol, hexane, or xylene, a light lubricant base oil or the like,with a polyamine, a succinic anhydride (derivative), and a boroncompound such as boric acid, a borate ester, or a borate salt, andheat-treating the resultant mixture under appropriate conditions.Thus-obtained boronated succinimide can usually have a boron contentfrom 0.1 to 4% by weight. In accordance with the present disclosure,boron-modified compounds of alkenylsuccinimide compounds (boronatedsuccinimides) may enable excellent heat resistance, antioxidantproperties and anti-wear properties to be achieved.

The content of boron contained in the boronated ashless dispersant isnot particularly limited. Usually, the content is from 0.1 to 3% byweight based on the weight of the ashless dispersant. In one embodimentof the present disclosure, the boron content in the ashless dispersantis 0.2% by weight or more, or 0.4% by weight or more, and is 2.5% byweight or less, 2.3% by weight or less, or 2.0% by weight or less. Theboronated ashless dispersant may be a boronated succinimide, such as aboronated bis-succinimide.

The boronated ashless dispersant has a weight ratio of boron to nitrogen(B/N ratio) of 0.1 or more, 0.2 or more, and less than 1.0, or 0.8 orless.

The content of the ashless dispersant may be adjusted as appropriateand, for example, is from 0.01 to 20% by weight, or from 0.1 to 10% byweight based on the total weight of the lubricant composition. When thecontent of the ashless dispersant is below the lower limit, the sludgedispersibility may be insufficient. On the other hand, when the contentis above the upper limit, the lubricant composition may deterioratecertain rubber materials, or deteriorate the low-temperature fluidity.

(E) Metal Detergent

Examples of the metal detergent include detergents containing an alkalimetal or an alkaline earth metal. Examples include, but are not limitedto, sulfonates containing an alkali metal or an alkaline earth metal,salicylates containing an alkali metal or an alkaline earth metal,phenates containing an alkali metal or an alkaline earth metal. Examplesof the alkali metal or alkaline earth metal include, but are not limitedto, magnesium, barium, sodium, and calcium.

More specifically, calcium sulfonate, magnesium sulfonate, calciumsalicylate, magnesium salicylate, calcium phenate, and magnesium phenateare used. The metal detergents may be used alone or in combination oftwo or more. The content of an alkali metal or alkaline earth metalcontained in the metal detergent is, but not limited to, from 0.1 to 20%by weight, from 0.5 to 15% by weight, or from 1.0 to 15% by weight.

The metal detergent has a total base number, but not limited to, from 20to 600 mgKOH/g, from 50 to 500 mgKOH/g, from 100 to 450 mgKOH/g, or from150 to 400 mgKOH/g. Such metal detergents allow the lubricantcomposition to have an acid neutralizing property, a high-temperaturedetergency, and an anti-rust property.

The metal detergent may be contained in the lubricant composition at anypercentage. For example, the percentage is from 0.01 to 5% by weight,from 0.1 to 4% by weight, or from 0.2 to 3% by weight.

In addition to the above, the lubricant composition of the presentdisclosure may further comprise other additives. Examples of the otheradditives include oily agents, rust preventives, antioxidants, extremepressure agents, corrosion inhibitors, metal deactivators, pour pointdepressants, anti-foaming agents, coloring agents, and additive packagesfor automatic transmission fluid. Additive packages for variouslubricants containing at least one of them can also be added. Whenadditives containing phosphorus are used, the total content ofphosphorus is adjusted to a range from 300 to 1500 ppm by weight, from400 to 1400 ppm by weight, from 500 to 1300 ppm by weight, from 600 to1200 ppm by weight, or from 600 to 1000 ppm by weight, based on thetotal weight of the lubricant composition.

The high temperature high shear viscosity (HTHS viscosity) at 150° C. ofthe lubricant composition of the present disclosure is, but not limitedto, from 1.4 to 2.9 mPa·s, or from 1.7 to 2.6 mPa·s.

The kinematic viscosity at 100° C. of the lubricant composition of thepresent disclosure is, but not limited to, from 3 to 9.3 mm²/s, from 3to 8.2 mm²/s, or from 4 to 8.2 mm²/s. When the kinematic viscosity at100° C. of the lubricant composition is below the lower limit, there isa possibility that a sufficient coefficient of friction cannot beobtained. On the other hand, when the kinematic viscosity at 100° C. isabove the upper limit, the viscosity resistance is increased, and thefuel consumption is worsened.

The viscosity index of the lubricant composition of the presentdisclosure is, but not limited to, 120 or more, or 160 or more. When theviscosity index of the lubricant composition is below the lower limit,there is a possibility that sufficient low-temperature characteristicscannot be obtained. The upper limit is, but not limited to, 250.

The lubricant composition of the present disclosure, though being madelow-viscosity, exhibits an excellent friction reducing effect, and alsohas an excellent wear resistance. The lubricant composition of thepresent disclosure is suitably used as an oil disposed between slidingsurfaces of sliding members. The lubricant composition of the presentdisclosure also suitably functions as a lubricant for diamond-likecarbon (DLC) films. In particular, the lubricant composition of thepresent disclosure can be disposed between a DLC-coated sliding surfaceof a sliding member, which has at least one sliding surface coated witha diamond-like carbon (DLC) film, and a sliding surface of the othermetal member (in particular, a steel material), so that the frictionbetween the sliding surfaces can be further reduced and an excellentwear resistance can be obtained. Thus, in some embodiments of thepresent disclosure, a diamond-like carbon (DLC) film and the lubricantcomposition are combined.

The diamond-like carbon (DLC) film in the present disclosure may be aDLC film having an amorphous structure. The DLC film is formed on atleast one sliding surface of sliding members. The DLC film can be dopedwith a predetermined element. Examples of the element include boron (B),titanium (Ti), vanadium (V), and molybdenum (Mo). In some embodiments,the element is boron. In other embodiments of the present disclosure, aDLC film doped with boron and the lubricant composition are combined, sothat both better friction reducing effect and wear resistance areobtained.

The content of boron is from 1 to 30%, or from 4 to 25% when the entireDLC film is considered as 100 atom %. When the content of boron is belowthe lower limit, insufficient friction reducing effect and wearresistance may be obtained. On the other hand, when the content of boronis above the upper limit, a good DLC film may not be formed.

The DLC film in the present disclosure may be a DLC not containinghydrogen, so-called hydrogen-free DLC, but a DLC containing hydrogen maybe used because the friction reducing effect can be easily obtained. Thecontent of hydrogen is from 0 to 25%, from 5 to 25%, from 10 to 22%, orfrom 15 to 20%, when the entire film is considered as 100 atom %. As thecontent of hydrogen in the DLC film increases, there is a tendency thatthe low-friction characteristics can be improved. However, when thecontent of hydrogen is excessive, the DLC film may be excessivelysoftened to have a reduced wear resistance.

The DLC film in the present disclosure may contain a modificationelement for improving the sliding characteristics and the like and/orinevitable impurities. Examples of the element include O, Al, Mn, Si,Cr, W, and Ni. The content of the element is not particularly limited,and may be adjusted within a range not impairing the effects of thepresent disclosure. In particular, the content is less than 8 atom %, orless than 4 atom %. The composition of the DLC film may be homogeneous,slightly changed, or even inclined, with respect to the thicknessdirection.

A substrate on which the DLC film is formed (i.e., a substrate of asliding member) is not particularly limited. The DLC film is harder thanthe substrate, and has a smaller modulus of elasticity than thesubstrate. Such substrates can improve the wear resistance, thetoughness, the impact resistance or the like of the DLC-coated surface.For example, the DLC film in the present disclosure has a hardness from10 to 30 GPa, or from 14 to 25 GPa. When the hardness is too low, thewear resistance is reduced. When the hardness is too high, the DLC filmis likely to occur cracks or the like. From the same viewpoint, forexample, the modulus of elasticity of the DLC film is from 100 to 200GPa, from 110 to 190 GPa, from 120 to 180 GPa, or from 130 to 170 GPa.

The DLC film may be formed according to a known method. For example, amethod described in Japanese Unexamined Patent Publication (Kokai) No.2014-224239 can be followed. Specifically, a dense DLC film can beefficiently formed by sputtering, or by unbalanced magnetron sputtering(UBMS). The inside of the chamber is evacuated to 10⁻⁵ Pa or less beforeformation of the DLC film, or a hydrogen gas is introduced into thechamber to remove the remaining oxygen and water in the chamber beforefilm formation. The amount of the hydrogen gas introduced may beadjusted according to the content of H in the DLC film.

As the sputtering gas, for example, one or more rare gases such as argon(Ar) gas, helium (He) gas, and nitrogen (N₂) gas can be used. As aH-containing gas, one or more hydrocarbon gases such as methane (CH₄),acetylene (C₂H₂), and benzene (C₆H₆) can be used. The gas flow rate, theDLC film formation temperature, and the like may be selected asappropriate according to known methods.

The lubricant composition of the present disclosure can be applied tosliding members in a wide variety of machines. In particular, a slidingmachine comprising the lubricant composition of the present disclosureand a sliding member coated with a DLC film (in particular, aboron-containing DLC film) has a very small coefficient of frictionbetween the sliding surfaces and an excellent wear resistance, and thusthe lubricant composition is suitable for applications where mechanicalloss due to sliding is to be reduced. For example, the lubricantcomposition can be applied to pistons, piston rings, piston pins, crankshafts, gears, rotors, rotor housings, cams, and valve lifters. Inparticular, the lubricant composition of the present disclosure can besuitably used for use in internal combustion engines.

EXAMPLES

The present disclosure will now be described in detail by showingExamples and Comparative Examples, but is not limited thereto.

Evaluation materials for the Block-on-Ring friction test shown belowwere prepared as test samples used as sliding members in the followingExamples and Comparative Examples.

As a substrate, a block-shaped (6.3 mm by 15.7 mm by 10.1 mm) steelmaterial (SUS440C) subjected to a quenching treatment was prepared. ADLC film was deposited on a mirror-finished surface (surface roughnessRzjis 0.1 μm/sliding surface) of the steel material using an unbalancedmagnetron sputtering apparatus (UBMS 504 manufactured by Kobe Steel,Ltd.). B4C was used as a doping target to deposit a boron-doped DLCfilm. Deposition of the DLC film by sputtering was done according to amethod described in Japanese Unexamined Patent Publication (Kokai) No.2014-224239. Thus, the evaluation material on which the boron-doped DLCfilm (containing hydrogen) was deposited was obtained. The thickness ofthe DLC film was about 2 μm. For the composition of the obtained film,the content of boron in the boron-doped DLC film was 6% as a boroncontent when the entire film was considered as 100 atom %.

As evaluation materials not coated with a DLC film, a standard testpiece made of a steel material (SAE4620) manufactured by FalexCorporation (hardness: RC58-63) was prepared as a ring test piece, and astandard test piece made of a steel material (SAE O1) manufactured byFalex Corporation (hardness: RC58-63) was also prepared as a block testpiece.

In the tables below, test pieces coated with a boron-doped DLC film arerepresented as “B-DLC,” and test pieces not coated with a DLC film(steel materials) are represented as “Steel.”

Lubricant compositions in Examples and Comparative Examples comprise thefollowing components.

Lubricant Base Oil a GTL-derived base oil having a kinematic viscosityat 100° C. of 4.1 mm²/s, and VI of 127

(A) Molybdenum Dialkyldithiophosphate (MoDTP)

a compound containing molybdenum at a content of 9% by weight, andrepresented by formula (1) above, wherein X₁ and X₂ are each an oxygenatom, Y₁ and Y₂ are each a sulfur atom, each R is a monovalent C₈hydrocarbon group

Friction Modifiers Other than (A) Above

a molybdenum dithiocarbamate MoDTC having a molybdenum content of 10% byweight

a trinuclear Mo compound having a molybdenum content of 5.5% by weight

oleylamine

oleamide

(B) Zinc Dialkyldithiophosphate

a Pri-ZnDTP (a zinc dialkyldithiophosphate having a primary alkyl group)

a Sec-ZnDTP (a zinc dialkyldithiophosphate having a secondary alkylgroup)

(B′) Acidic Phosphate Ester

a mixture of compounds represented by following formula (6), wherein R⁶is a 2-ethylhexyl group, and n is 1 or 2

(C) Metal Detergent

Ca salicylate having a total base number of 180 mgKOH/g, and a Cacontent of 6.3% by weight

Mg sulfonate having a total base number of 400 mgKOH/g, and a Mg contentof 9.4% by weight

(D) Ashless Dispersant

boronated succinimide having a B content of 0.7% by weight, and a Ncontent of 2.0% by weight

non-boronated succinimide having a N content of 1.0% by weight

(E) Viscosity Index Improver

polymethacrylate having Mw of 300,000

(F) Other Additive Packages

Antioxidants: phenolic antioxidant and amine antioxidant

Anti-foaming agent: dimethyl silicone

Examples 1 to 14 and Comparative Examples 1 to 5

Lubricant compositions were prepared by mixing the components describedabove at the compositions and the contents described in the tables.

The contents shown in the tables are described below.

The content of a molybdenum friction modifier is in terms of ppm byweight of molybdenum based on the total amount of the lubricantcomposition. For a MoDTP, the content of phosphorus derived from theMoDTP in ppm by weight based on the total amount of the lubricantcomposition is also shown.

The content of a zinc dialkyldithiophosphate is in terms of ppm byweight of phosphorus derived from the zinc dialkyldithiophosphate basedon the total amount of the lubricant composition.

The contents of oleylamine, oleamide, and an acidic phosphate ester arein % by weight based on the total amount of the lubricant composition.

The contents of metal detergents are in terms of % by weight of calciumand magnesium based on the total amount of the lubricant composition.

The content of an ashless dispersant is shown in terms of ppm by weightof boron based on the total amount of the lubricant composition, and interms of ppm by weight of nitrogen based on the total amount of thelubricant composition.

The tables also show the total content of phosphorus in ppm by weightbased on the total amount of the lubricant composition.

These lubricant compositions were tested as described below. The resultsare shown in the tables.

(1) The kinematic viscosity at 100° C. (KV100) was measured according toASTM D445.(2) The high temperature high shear viscosity (HTHS viscosity) at 150°C. was measured according to ASTM D4683.

(3) Determination of Minimum Coefficient of Friction

A Block-on-Ring friction test was carried out using a block test piecehaving a sliding surface width of 6 mm (a test piece coated with a DLCfilm or a test piece not coated with a DLC film (a standard test piecemade of a steel material (SAE O1) manufactured by Falex Corporation(hardness: RC58-63)); and as a counterpart, a ring test piece made of asteel material having an outer diameter of 35 mm and a width of 9 mm (astandard test piece made of a steel material (SAE4620) manufactured byFalex Corporation (hardness: RC58-63)). An embodiment of theBlock-on-Ring friction test is schematically depicted in FIG. 1. ABlock-on-Ring friction test was conducted for 30 minutes with a testload of 294 N, a sliding velocity of 0.3 m/s, and an oil temperature of80° C. (constant), and the minimum coefficient of friction during 30minutes was taken as the minimum coefficient of friction in this test.

(4) Evaluation of Wear Amount

Block test pieces before and after the Block-on-Ring friction test weremeasured for the surface roughness on a surface roughness tester(SURFTEST SV-3200 manufactured by Mitutoyo Corporation), and the wearamount was determined. For the wear amount, measurements were made at atotal of three places, one at the central portion of the sliding markand two portions at 1 mm from the both ends toward the central portionof the sliding mark, and the average value was taken as the wear amountin this test.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Example 7 Example 8 lubricant base oil balance balance balance balancebalance balance balance balance Mo friction MoDTP (Mo) 700 700 700 700700 700 700 700 modifier ppm by weight (P) 250 250 250 250 250 250 250250 ppm by weight ZnDTP Primary ZnDTP ppm by weight 300 200 750 — — 300300 300 (P content) Secondary ZnDTP ppm by weight 450 300 — 750 450 450450 450 P content total ppm by weight 750 500 750 750 450 750 750 750metal detergent Ca salicylate % by weight 0.14 0.14 0.14 0.14 0.14 0.14— 0.20 (metal element Mg sulfonate % by weight 0.06 0.06 0.06 0.06 0.06— 0.06 — content) dispersant B dispersant ppm by weight 600/200 600/200600/200 600/200 600/200 600/200 600/200 600/200 (N content/B non-Bdispersant ppm by weight 200/0  200/0  200/0  200/0  200/0  200/0 200/0  200/0  content) viscosity index improver % by weight 1 1 1 1 1 11 1 other additives % by weight 1 1 1 1 1 1 1 1 Mo content in the wholecomposition ppm by weight 700 700 700 700 700 700 700 700 P content inthe whole composition ppm by weight 1000 750 1000 1000 700 1000 10001000 evaluation KV100 mm²/s 6.5 6.5 6.5 6.7 6.5 6.5 6.2 6.6 resultsHTHS150 mPa · s 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 B- minimum — 0.042 0.0420.041 0.041 0.041 0.044 0.045 0.046 DLC/ coefficient Steel of frictionwear amount μm 0.27 0.40 0.67 0.33 0.46 0.50 0.32 0.55 Steel/ minimum —0.057 0.059 0.054 0.051 0.053 0.041 0.059 0.052 Steel coefficient offriction wear amount μm 1.00 0.90 0.73 0.37 0.73 0.63 0.50 0.93

TABLE 2 Example 9 Example 10 Example 11 Example 12 Example 13 lubricantbase oil balance balance balance balance balance Mo friction MoDTP (Mo)700 1000 700 650 600 modifier ppm by weight (P) 250 360 250 230 210 ppmby weight Mo-trimer (Mo) — — — — — ppm by weight MoDTC (Mo) — — — 50 100ppm by weight ZnDTP Primary ZnDTP ppm by weight 300 200 300 200 200 (Pcontent) Secondary ZnDTP ppm by weight 450 300 450 300 300 P contenttotal ppm by weight 750 500 750 500 500 metal detergent Ca salicylate %by weight — 0.14 0.14 0.14 0.14 (metal element Mg sulfonate % by weight0.20 0.06 0.06 0.06 0.06 content) dispersant B dispersant ppm by weight600/200 600/200 — 600/200 600/200 (N content/B non-B dispersant ppm byweight 200/0  200/0  500/0 200/0  200/0  content) viscosity indeximprover % by weight 1 1 1 1 1 other additives % by weight 1 1 1 1 1 Mocontent in the whole composition ppm by weight 700 1000 700 700 700 Pcontent in the whole composition ppm by weight 1000 860 1000 730 710evaluation KV100 mm²/s 6.4 6.5 6.7 6.6 6.5 results HTHS150 mPa · s 2.32.3 2.3 2.3 2.3 B-DLC/Steel minimum — 0.047 0.045 0.046 0.046 0.048coefficient of friction wear amount μm 0.37 0.47 0.47 0.43 0.70Steel/Steel minimum — 0.061 0.049 0.061 0.053 0.051 coefficient offriction wear amount μm 0.73 0.67 0.57 0.57 0.67

TABLE 3 Comparative Comparative Comparative Comparative Example 1Example 2 Example 3 Example 4 lubricant base oil balance balance balancebalance Mo friction MoDTP (Mo) — — — 1400 modifier ppm by weight (P) 500ppm by weight Mo-trimer (Mo) 700 — 100 — ppm by weight MoDTC (Mo) — 700— — ppm by weight ZnDTP Primary ZnDTP ppm by 300 300 300 300 (P content)weight Secondary ZnDTP ppm by 450 450 450 450 weight P content total ppmby 750 750 750 750 weight metal Ca salicylate % by weight 0.14 0.14 0.140.14 detergent Mg sulfonate % by weight 0.06 0.06 0.06 0.06 (metalelement content) dispersant B dispersant ppm by 600/200 600/200 600/200600/200 (N content/B weight content) non-B dispersant ppm by 200/0 200/0  200/0  200/0  weight viscosity index improver % by weight 1 1 1 1oleylamine % by weight — — — — acidic phosphate ester % by weight — — —— oleamide % by weight — — — — other additives % by weight 1 1 1 1 Mocontent in the whole composition ppm by 700 700 100 1400 weight Pcontent in the whole composition ppm by 750 750 750 1250 weightevaluation KV100 mm²/s 6.6 6.6 6.6 6.5 results HTHS150 mPa · s 2.3 2.32.3 2.3 B- minimum — 0.055 0.051 0.052 0.047 DLC/Steel coefficient offriction wear μm 0.93 1.57 0.57 0.77 amount Steel/Steel minimum — 0.0650.058 0.088 — coefficient of friction wear μm 0.63 0.93 0.80 — amountComparative Comparative Comparative Comparative Example 5 Example 6Example 7 Example 8 lubricant base oil balance balance balance balanceMo friction MoDTP (Mo) 350 700 700 700 modifier ppm by weight (P) 125250 250 250 ppm by weight Mo-trimer (Mo) — — — — ppm by weight MoDTC(Mo) — — — — ppm by weight ZnDTP Primary ZnDTP ppm by 300 300 300 300 (Pcontent) weight Secondary ZnDTP ppm by 450 450 450 450 weight P contenttotal ppm by 750 750 750 750 weight metal Ca salicylate % by weight 0.140.14 0.14 0.14 detergent Mg sulfonate % by weight 0.06 0.06 0.06 0.06(metal element content) dispersant B dispersant ppm by 600/200 600/200600/200 600/200 (N content/B weight content) non-B dispersant ppm by200/0  200/0  200/0  200/0  weight viscosity index improver % by weight1 1 1 1 oleylamine % by weight — 0.19 0.19 — acidic phosphate ester % byweight — — 0.16 — oleamide % by weight — — — 0.19 other additives % byweight 1 1 1 1 Mo content in the whole composition ppm by 350 700 700700 weight P content in the whole composition ppm by 875 1000 1200 1000weight evaluation KV100 mm²/s 6.5 6.6 6.6 6.6 results HTHS150 mPa · s2.3 2.3 2.3 2.3 B- minimum — 0.099 0.054 0.059 0.061 DLC/Steelcoefficient of friction wear μm 0.13 0.83 1.50 1.07 amount Steel/Steelminimum — — — — — coefficient of friction wear μm — — — — amount

As shown in Comparative Example 2, the lubricant composition comprisinga molybdenum dithiocarbamate (MoDTC) alone provided an insufficienteffect of reducing the friction and caused a significant wear betweenthe sliding surfaces of the DLC-coated surface and the steel material.As shown in Comparative Example 1, the lubricant composition comprisinga trinuclear molybdenum compound alone resulted in increased frictionbetween the sliding surfaces. Further, the wear between the slidingsurfaces of the DLC-coated surface and the steel material was increased.As shown in Comparative Example 3, even where the content of atrinuclear molybdenum compound in the lubricant composition comprising atrinuclear molybdenum compound alone was decreased, the friction betweenthe sliding surfaces was still large. In addition, as shown inComparative Examples 6 to 8, the lubricant compositions comprising anamino friction modifier or an amide friction modifier at a contentdescribed in Example in Japanese Unexamined Patent Publication (Kokai)No. 2016-216653 (Patent Literature 3) resulted in high coefficients offriction between the sliding surfaces of the B-DLC-coated surface andthe steel material, and the wear amount was large.

Compared to them, as shown in Tables 1 and 2, the lubricant compositionof the present disclosure possibly gave excellent friction reducingeffects not only between the sliding surfaces of the steel material andthe steel material but also between the sliding surfaces of theB-DLC-coated surface and the steel material, and possibly improved thewear resistance.

INDUSTRIAL APPLICABILITY

In particular, the lubricant composition of the present disclosure canprovide sliding surfaces between a DLC-coated surface and another metalmember (in particular, a steel material) with a lower coefficient offriction than a lubricant composition only comprising a molybdenumdithiocarbamate or a trinuclear Mo compound as a friction modifier, andalso is excellent in wear resistance. The lubricant composition of thepresent disclosure is suitably used as a lubricant compositionparticularly for use in internal combustion engines.

REFERENCE SIGNS LIST

-   1: Load-   2: Block test piece-   3: Ring test piece-   4: Lubricant composition

1. A lubricant composition comprising a lubricant base oil, (A) amolybdenum dialkyldithiophosphate, and (B) a zincdialkyldithiophosphate, and having a phosphorus content of 300 to 1500ppm by weight based on a total weight of the lubricant composition. 2.The lubricant composition according to claim 1, wherein the lubricantcomposition further comprises (A′) at least one selected from the groupconsisting of an amine friction modifier and an amide friction modifierat a content of less than 0.1% by weight based on the total weight ofthe lubricant composition.
 3. The lubricant composition according toclaim 1, comprising component (A) at a content of 400 to 1300 ppm byweight in terms of ppm by weight of molybdenum based on the total weightof the lubricant composition, and component (B) at a content of 200 to1000 ppm by weight in terms of ppm by weight of phosphorus based on thetotal weight of the lubricant composition.
 4. The lubricant compositionaccording to claim 1, for use in lubrication of diamond-like carbon(DLC) films.
 5. The lubricant composition according to claim 1, for usein lubrication of opposed sliding surfaces of sliding members, whereinthe sliding members are a pair of sliding members having opposed slidingsurfaces which can move relative to each other, and wherein at least oneof the sliding surfaces comprises a surface coated with a diamond-likecarbon (DLC) film.
 6. The lubricant composition according to claim 4,wherein the DLC film comprises boron.
 7. The lubricant compositionaccording to claim 1, wherein the lubricant composition has a hightemperature high shear viscosity (HTHS viscosity) at 150° C. of 1.4 to2.9 mPa·s.
 8. The lubricant composition according to claim 1, whereinthe lubricant composition has a kinematic viscosity at 100° C. of 9.3mm²/s or less.
 9. The lubricant composition according to claim 1, foruse in internal combustion engines.
 10. The lubricant compositionaccording to claim 1, wherein the lubricant composition does notcomprise (A′) at least one selected from the group consisting of anamine friction modifier and an amide friction modifier.
 11. Thelubricant composition according to claim 2, comprising component (A) ata content of 400 to 1300 ppm by weight in terms of ppm by weight ofmolybdenum based on the total weight of the lubricant composition, andcomponent (B) at a content of 200 to 1000 ppm by weight in terms of ppmby weight of phosphorus based on the total weight of the lubricantcomposition.
 12. The lubricant composition according to claim 2, whereinthe lubricant composition has a high temperature high shear viscosity(HTHS viscosity) at 150° C. of 1.4 to 2.9 mPa·s.
 13. The lubricantcomposition according to claim 3, wherein the lubricant composition hasa high temperature high shear viscosity (HTHS viscosity) at 150° C. of1.4 to 2.9 mPa·s.
 14. The lubricant composition according to claim 4,wherein the lubricant composition has a high temperature high shearviscosity (HTHS viscosity) at 150° C. of 1.4 to 2.9 mPa·s.
 15. Thelubricant composition according to claim 5, wherein the lubricantcomposition has a high temperature high shear viscosity (HTHS viscosity)at 150° C. of 1.4 to 2.9 mPa·s.
 16. The lubricant composition accordingto claim 6, wherein the lubricant composition has a high temperaturehigh shear viscosity (HTHS viscosity) at 150° C. of 1.4 to 2.9 mPa·s.17. The lubricant composition according to claim 2, wherein thelubricant composition has a kinematic viscosity at 100° C. of 9.3 mm²/sor less.
 18. The lubricant composition according to claim 3, wherein thelubricant composition has a kinematic viscosity at 100° C. of 9.3 mm²/sor less.
 19. The lubricant composition according to claim 4, wherein thelubricant composition has a kinematic viscosity at 100° C. of 9.3 mm²/sor less.
 20. The lubricant composition according to claim 5, wherein thelubricant composition has a kinematic viscosity at 100° C. of 9.3 mm²/sor less.